New Ford Kuga

The answer must surely depend on what you want to tow .....☺

We had our 2Ltr mild hybrid (self charging) back in August 2020 which we use to tow our 2015 Swift challenger sport 554 and are very pleased with it, last month we went to the new forest centenary site via Birdlip hill and we flew up the 8.6% gradient plus we had the factory fitted electric tow bar fitted  which in fact worked out cheaper than having an after market manual fitting one 

I wondered where you'd got that gradient from ..... I've driven up & down Birdlip Hill many times .... seen the aftermath of many crashes too, there was a time that the white lines allowed you to overtake going down hill too. 

It seems the average gradient is 8.6% .... https://www.climbbybike.com/climb/Birdlip-Hill/8995

The real killer is on the right turn on the island at the top by the aptly named pub the "Air Balloon" according the internet the steepest part is 16% if you get the chance there is place to park and take in the views over Gloucester